The invention relates to a DC motor having a stator unit and a rotor unit, which is connected to an electrically conductive shaft in a rotationally fixed manner, said shaft being rotatably mounted in at least one stationary sintered bearing. The DC motor also has a shielding for protection against high-frequency electromagnetic fields.
A generic DC motor is known, for example, from the German patent application DE 1 638 216 A1. It is also thereby known to provide a metal plate, which is electrically connected to ground, on the end face of the stator unit which is opposite the rotor unit, i.e. in the region of a base plate or mounting plate of the stator unit, in order to provide shielding in this axial direction.
The present invention is based on a brushless, electronically commutated DC motor (EC=electrically commutated or BLDC=brushless direct current) of flat construction in which the axial expansion of the motor is therefore small in relation to the diameter thereof. This is substantially achieved by the known external rotor design in which the rotor unit wraps around the outside of the stator unit.
Conventional BLDC motors with small capacity emit electromagnetic waves via the windings thereof during the motor operation. These radiated waves can be measured within the scope of EMC tests and are undesirable in many technical environments, for example in motor vehicles, which include other electrical devices that are susceptible to interference. In order to reduce said radiated waves, metallic shieldings, in particular the underside metallic plate that was previously mentioned with regard to the generic DC motor, can be mounted in close proximity to the windings. In the case of the external rotor, the further shielding on the opposite side can be implemented by a metallic rotor mounted in close proximity to the stator. In order to achieve an effective shielding, it is not sufficient to surround the components to be shielded with conductors. Said components have to be contacted to ground. Otherwise, in the worst case, the shielding itself can couple or decouple high-frequency energy while functioning like an antenna and thereby worsen the interference problem. The electrical connection to the rotating rotor shielding required here is however not easily produced. It can, in fact, occur in a known manner via a sliding contact, which however requires additional components and presents a problem with regard to service life.
The German patent application DE 10 2007 019 431 A1 discloses interference suppression measures for a brush motor, within the scope of which the rotor shaft is galvanically coupled via a bearing of the shaft and a connection means for contacting the bearing to a reference potential, in particular a supply potential, in an electrically conductive manner.
The German patent application DE 10 2006 024 551 A1 discloses a method for shielding an electronic device, in particular a circuit, which is surrounded by a conductive cap, in which planar regions of the cap are arranged in an electrically insulated manner at a small distance from and parallel to a planar region of a conductor of a reference potential; thus enabling a capacitive coupling of the cap to the reference potential to occur.